In order to fulfil current stringent emission legislation more or less all vehicles with internal combustion engines are provided with an exhaust gas aftertreatment device comprising at least one catalytic converter substrate. A catalytic substrate generally comprises a channeled structure which exhaust gas can pass through while being exposed to the large surface area of the catalytic substrate. The channels of the catalytic substrates may be fluidly connected by perforating holes or like allowing gas to pass between adjacent channels. This enables gas to diffuse through the catalytic substrate structure. For petrol engines the most frequently used catalytic substrates are of Three Way Catalyst (TWC) type, while catalytic substrates of Diesel Oxidation Catalyst (DOC) type and/or Lean NOx Trap (LNT) type are the most frequently used catalytic substrates for diesel engines. It is also preferred that the TWC or the DOC/LNT is supplemented by a catalytic substrate with Selective Catalytic Reduction (SCR) functionality for improved NOx reduction. Typically, when using a catalytic substrate of SCR type a liquid or gaseous reductant is added to the exhaust gas emission flow before the exhaust gas enters the catalytic substrate of SCR type. The addition of reductant enables the catalytic reduction were NOx is reduced to diatomic nitrogen, N2, and water, H2O. Catalytic substrates combining the functionalities of more than one type of catalytic substrate in one catalytic substrate also exist.
Combining more than one catalytic substrate can be problematic since exhaust gas aftertreatment devices often are associated with design restrains due to the limited available space in the engine compartment. Thus, small exhaust gas aftertreatment devices are preferred from an engine packaging perspective, but small exhaust gas aftertreatment devices usually means that the flow distance between the inlet and the catalytic substrates of the exhaust gas aftertreatment device is limited. Limited distance means that the time and distance during which mixing of the exhaust gas emissions can occur is limited. Insufficient mixing of the exhaust gas emissions gives inhomogeneous exhaust gas emission mixture. This might e.g., be problematic for emission gas sensors, arranged in the exhaust gas emission flow, to work properly and give accurate emission measurements.
It is also desirable that positioning of the inlet and outlet of the exhaust gas aftertreatment device is flexible and that the inlet and outlet not necessarily have to be aligned. Compact design and flexible positioning of the inlet and outlet enables optimized and minimized packing volume.
Other problematic areas for exhaust gas aftertreatment devices are high back pressure and insufficient heating. High back pressure implies significant exhaust gas flow resistance. This is negative for the efficiency of the combustion engine resulting in a decrease of power output. Compensation of such decrease in power output leads to an increase in fuel consumption. If there is a difference in back pressure between two possible flow paths the flow through the flow path with lowest backpressure will be larger than the flow through the flow path with the higher backpressure. The flow ratio will be in proportion to the difference in back pressure. Heating of the catalytic substrate is crucial since the catalytic substrate is most effective at relatively high temperatures. Thus, it is desirable that the catalytic substrate reaches its optimum operation temperature as soon as possible and that the catalytic substrate stays warm during operation.
Insufficient mixing of the exhaust gas emissions are of particular interest if an exhaust gas aftertreatment device with a catalytic substrate with SCR functionality is used. When a liquid reductant is used it is also desirable that the liquid reductant is evaporated. Consequently, sufficient mixing and reductant evaporation is essential for the catalytic substrate with SCR functionality to work properly. A homogenous exhaust gas and reductant mix, and suitable ratio between exhaust gas and injected reductant, is also beneficial for the catalytic properties of catalytic substrates of SCR type. The mixing of injected reductant and exhaust gas is benefited by long distance between the reductant injecting device and the catalytic substrate of SCR type. The evaporation of injected reductant is also benefited by long distance between the injection of reductant and the subsequently provided catalytic converter of SCR type such that the injected reductant can be exposed to hot exhaust gas for a longer period of time. Thus, compact design directly contradicts other desirable properties of the exhaust gas aftertreatment device such as sufficient mixing.
US 2008/0041036 A1 discloses a method for adding at least one, in particular liquid, reactant to an exhaust gas stream of an internal combustion engine and a device for treating an exhaust gas stream of an internal combustion engine. According to US 2008/0041036 A1 an element, wherein the element may be a particulate filter or catalyst body, is arranged in an exhaust gas flow. Downstream of the element a nozzle may be provided spraying reactant flow onto the element such that a coating covering preferably at least 10% of the length of the element is formed. According to at least one embodiment the exhaust gas flow subsequently is redirected at a reversal region such that the exhaust gas flow is redirected to flow substantially opposite the entering exhaust gas flow before being discharged from the device. The device disclosed in US 2008/0041036 A1 has many advantages but is only provided with one catalytic substrate. Additionally US 2008/0041036 A1 fails to provide sufficient mixing of reactant and exhaust gas.
Thus, there is a need for further improvements.